Although patients and physicians have shown tremendous interest in the effect of diet on ulcerative colitis, there is a lack of significant evidence for providers to make practical recommendations with. In this study, the investigators hope to find out if dietary therapy by either the Specific Carbohydrate Diet (SCD) or the Mediterranean diet will help improve ulcerative colitis symptoms for patients with mild to moderately active disease. In addition, the investigators will compare disease activity and changes in the intestinal bacterial composition in the colon that occur with the Mediterranean or the SCD diet in active ulcerative colitis.

This study is proposed as a single-site randomized trial consisting of 10 study visits to Massachusetts General Hospital (MGH) over 12 weeks. Participants in this study will be randomly assigned to the SCD or Mediterranean diet. The investigators ask that participants exclusively consume their assigned diet for 6 weeks, with all meals and snacks prepared by the metabolic kitchen within MGH. Participants will need to pick up food from MGH every 5-7 days, and will meet with a study dietitian before they begin and weekly during the diet therapy.

There will be a screening visit to determine eligibility for the study, as well as study visits at weeks 0, 1, 2, 4, 6, and a 10 week follow-up at MGH, in which participants will fill out questionnaires. Participants will need to provide stool samples at screening, week 6, and week 10. In addition, blood will be drawn at week 0 and week 6, and if participants are getting a clinically-indicated colonoscopy at the time of screening, up to eight research biopsies may be collected during the procedure.

This randomized, parallel-group feeding study will examine the influence of Mediterranean and SCD diets on gut microbiota, luminal inflammation, and disease-specific clinical indices in patients with mild to moderate ulcerative colitis (UC). 50 patients will be randomized to follow either the SCD or Mediterranean diet in a 1:1 ratio. Once informed consent is obtained, subjects will enter a 2-week screening period. Eligible subjects will be enrolled in the feeding treatment for 6 weeks followed by an additional follow-up visit at 10 weeks (4 weeks after feeding treatment completion). The total time to complete the study is 12 weeks. The metabolic kitchen within the Metabolism and Nutrition Research Center at MGH Translational and Clinical Research (TCRC) Unit will be responsible for preparing and packaging food for participants to pick up, as well as performing all necessary nutritional assessments. Participants will be provided with 3 meals and 2 snacks a day during the 6-week dietary intervention. Patient menus will be identical in each diet and will rotate on a 3-day basis. Total calories for each diet will be tailored according to each participant's body composition.

Inclusion Criteria:

• History of active ulcerative colitis for at least 3 months before screening
• Ulcerative colitis confirmed by colonoscopy or flexible sigmoidoscopy within 2 years of screening
• Mild to moderate ulcerative colitis at the time of screening (2 < Mayo score < 12)
• 1 ≤ Endoscopy subscore ≤ 2, or fecal calprotectin > 150 mcg/g within 2 weeks of screening
• Patients on 5-aminosalicylates (e.g. mesalamine, etc.) must be on a stable dose for ≥ 4 weeks prior to screening
• Patients on treatment with immunosuppressive (azathioprine/6-mercaptopurine and methotrexate) or biologic medications (infliximab, adalimumab, and golimumab) must be on stable dose for 8 weeks prior to baseline
• At the time of baseline, patients may be on no more than 20 mg of prednisone and 9 mg of budesonide MMX

Exclusion Criteria:

• Patients with Crohn's disease or indeterminate colitis
• History of colectomy
• Presence of ileal pouch or ostomy
• History of colonic dysplasia
• Evidence of active bacterial or viral gastroenteritis as indicated by positive stool studies for ova & parasites, clostridium difficile, and stool culture
• Severe to fulminant colitis
• Recent hospitalizations (within 2 weeks of screening) for ulcerative colitis requiring IV steroids
• Recent systemic antibiotics use (within 2 weeks of screening)
• Presence of the following labs indicative of severe colitis: a. Hemoglobin < 8.0 g/dl b. Albumin < 3.0 g/dl
• Use of Total Parenteral Nutrition (TPN)
• Active use of anti-diarrheal medications
• Use of cyclosporine, tacrolimus, or thalidomide within 2 months prior to screening

Only de-identified information may be shared with other collaborators and entities involved in generating data.

The investigators, subjects, and remaining study staff will not be intentionally unblinded. Due to the nature of the study, subjects, study staff, and investigators interacting with the subjects will likely be able to figure out what diet the subject is assigned to, however, the exact diet will not be explicitly revealed to them.